Electrical utilities must burn increasing quantities of fossil fuels to satisfy the ever-increasing demand for electric power. At the same time, electric utilities face increasing clean-air standards that are imposed upon their operation. In trying to satisfy the divergent demands of increasing power and decreased air pollution, electrical utilities have turned to using low-sulfur coals to fire their boilers and generate the steam needed for electrical power generation.
Electrical utilities have long relied upon electrostatic means such as electrostatic precipitators to remove particulate matter from boiler flue gas. The efficiency of operation of the electrostatic precipitators in the removal of particulate matter from boiler flue gas is dependent, in part, upon the electrical resistivity of the entrained particulate matter in boiler flue gas. It has been found that where a boiler is fired with low-sulfur coals, that is, coals having less than one percent sulfur content, the entrained particulate matter in the boiler flue gas has a high resistivity, for example, 10.sup.13 ohm-cm resistance and more. It has also been determined that the most efficient removal of particulate matter by electrostatic precipitation occurs when its resistivity is on the order of about 10.sup.8 ohm-cm and that when the resistivity of the particulate matter is higher, for example, on the order of 10.sup.13 ohm-cm, the efficiency of electrostatic precipitation is substantially reduced. Thus, reduced efficiency in the operation of electrostatic precipitators with the flue gas from low-sulfur coals has been attributed to the higher resistivity of such flue gas particles. Any reduction of the ability of an electrostatic precipitator to remove particles from the flue gas can offset, of course, the reduced or potentially reduced air pollution sought through the use of the more expensive low-sulfur coals.
One solution to this problem has been to condition the boiler flue gas prior to its entrance into the electrostatic precipitator by the use of a conditioning agent to reduce the resistivity of the entrained particles within the boiler flue gas. Among the various chemicals which have been used as conditioning agents for boiler flue gas are water, anhydrous ammonia and various ammonia-bearing solutions, sulfuric acid, sulfur trioxide and phosphoric acid.
U.S. Pat. No. 2,864,456 discloses an automatic control for electrostatic precipitators which varies both the electrostatic precipitator voltage and the supply of a conditioning agent such as water for particles to be removed by the electrostatic precipitator, to maintain an optimum sparking rate for efficient particle removal.
U.S. Pat. No. 3,284,990 discloses a method of improving the electrostatic precipitation of particles by adding phosphorous pentoxide to the particles prior to their electrostatic precipitation.
U.S. Pat. No. 3,523,407 discloses a method of improving the electrostatic precipitation of particles from a flue gas by adding preselected amounts of ammonia and water to the flue gas.
U.S. Pat. No. 3,665,676 discloses a system to condition the particles of boiler flue gas by the use of a salt solution such as a solution of ammonium sulfate or ammonium bisulfate. The salt solution is injected into the flue gas prior to entering the electrostatic precipitator and the system includes a metering means for controlling the amount of conditioner injected into the flue gas. U.S. Pat. No. 3,665,676 indicates that, if desired, conventional automatic controls can be provided to open the metering means when the flue gas reaches the desired operating temperature or to close it should the temperature fall below operating temperature. In addition, automatic controls can also be made to open the metering means to provide the amount of conditioner needed in proportion to the volume of gas to be conditioned.
U.S. Pat. No. 3,689,213 discloses a process for treating flue gas in which gaseous sulfur trioxide is generated in the immediate vicinity of the point of use as required by the quantity of fossil fuel being burned per unit time and is then introduced into the flue gas at a predetermined rate to facilitate fly ash removal by an electrostatic precipitator. In the system of U.S. Pat. No. 3,689,213, air and gaseous sulfur dioxide are heated in a heat exchanger to a temperature required for oxidation of sulfur dioxide to sulfur trioxide. The air and sulfur dioxide are passed through a catalytic converter for conversion of the sulfur dioxide to sulfur trioxide prior to its injection into the boiler flue gas.
U.S Pat. No. 3,722,178 discloses a system for the production of sulfur trioxide for flue gas conditioning including means to deliver a source of sulfur such as sulfuric acid to a vaporizer in proportion to the amount of flue gas from the boiler measured in terms of the electrical output generated at a particular time. As the production of flue gas changes in the boiler system, the proper ratio of acid to flue gas is automatically maintained by a control responsive to a signal coming from a boiler capacity index gauge to control the volume of sulfur trioxide being produced. The vaporizer is provided with a mixture of fresh air and a combustion gas from a natural gas or oil to convert the sulfuric acid to sulfur trioxide. The amount of combustion gas directed into the combustion chamber is automatically controlled by the exit temperature of the sulfur trioxide as indicated by temperature controllers mounted at the top and bottom of the vaporizer in the path of the output gas. The temperature controllers maintain the temperature of the vaporizer in the range for efficient production of sulfur trioxide. An additional temperature controller at the exit of the vaporizer turns off the burner when the temperature at the exit exceeds 1200.degree. F. (649.degree. C.).
More recent developments have centered on sulfur trioxide as a flue gas-conditioning material. Such flue gas-conditioning systems have included systems which store liquefied sulfur which is fed to a sulfur burner in which the sulfur is converted by combustion predominantly to sulfur dioxide. The systems then pass the sulfur dioxide to a catalytic converter which employs a vanadium pentoxide catalyst to convert the sulfur dioxide into sulfur trioxide. The sulfur trioxide created by such systems is piped to a nozzle system for injection into ducts carrying the boiler flue gas and its entrained particulate material to reduce the electrical resistivity of the flue gas particulate matter for removal by an electrostatic precipitator.
As reported in "Sulfur Trioxide Conditioning", Journal of the Air Pollution Control Association, Vol. 25, No. 2, February 1975, pp. 156-158, such systems have been in commercial use since 1972.
A number of prior systems have been disclosed to control such SO.sub.3 flue gas-conditioning systems. Such a system is disclosed, for example, in U.S. Pat. No. 3,993,429. In the system of U.S. Pat. No. 3,993,429 and in commercial systems resulting from this patent, a flow of heated air is forced into the sulfur burner; the temperature of the gas leaving the sulfur burner is sensed; and the sensed output temperature of the sulfur burner is used to control either the temperature of a flow of air forced into the sulfur burner, or the portion of a flow of heated air that is forced into the sulfur burner. The system of U.S. Pat. No. 3,993,429 increases or decreases the temperature of the air directed into the sulfur burner, or the portion of the heated air directed into the sulfur burner, in the event the burner outlet temperature is too low or too high, respectively. The system of U.S. Pat. No. 3,993,429 thus attempts to regulate the operating temperature of the sulfur burner and the catalytic converter downstream of the sulfur burner by regulating an air heater or an air flow diverter valve, or both, upstream of the sulfur burner. U.S. Pat. No. 3,993,429 also discloses a system in which the temperature of operation of the catalytic converter is controlled by providing a second flow of air to be mixed with the output of the sulfur burner, detecting the temperature of the mixture of the second flow of air and the gases leaving the sulfur burner and varying the temperature of the air in the second flow of air to maintain a desired operating temperature for the catalytic converter. U.S. Pat. No. 3,993,429 further discloses that SO.sub.3 flue gas-conditioning systems can operate by sensing the rate of coal combustion and varying the rate of flow of sulfur into a sulfur burner in response to the rate of coal combustion.
U.S. Pat. No. 4,284,417 discloses a system for regulating electric power supplied to the corona-generating electrodes of an electrostatic precipitator in response to changes in opacity of the flue gas exiting from the precipitator to control and minimize electric power consumption. In the system of U.S. Pat. No. 4,284,417, an output of an opacity transducer, which is a measure of the opacity of the flue gas, is directed to a controller for the electric power supplied to the corona-generating electrodes of the electrostatic precipitator. If the opacity of the flue gas exceeds a high opacity limit set in the controller, the controller increases the power to the corona-generating electrodes; and if the opacity of the flue gas is less than the low opacity limit, the controller decreases the power to the corona-generating electrodes.
U.S. Pat. No. 4,624,685 discloses a system for optimizing the power consumption of an electrostatic precipitator. The system of U.S. Pat. No. 4,624,685 includes a controller for the transformer-rectifier sets of the electrostatic precipitator that determines the corona power required to reduce flue gas particulate matter below the environmental limit from a load indexed transducer, data input to the system and stored data and algorithms. The precipitator power is then reduced or trimmed in response to an average measured opacity of the flue gas to provide minimal precipitation power consumption consistent with meeting the environmental limit.
U.S. Pat. No. 4,770,674 discloses a system for conditioning flue gas for an electrostatic precipitator, including equipment for converting sulfur into sulfur trioxide. The disclosed systems of U.S. Pat. No. 4,770,674 include a sulfur burner to produce oxidized sulfur, a catalytic converter to convert the oxidized sulfur to sulfur trioxide, and means to control sulfur and air inputs to the sulfur burner. Various inputs to the control means are disclosed, including the outlet temperature of the catalytic converter and such operating parameters of the exhaust stage of the system as the output temperature of the exhaust gas from the precipitator, the flow rate of the exhaust gas, the power delivered to or the speed of, an induced draft fan, if any, the opacity of the exhaust gas within the stack, and the power dissipated by the precipitator.
U.S. Pat. No. 4,779,207 discloses a system for preconditioning flue gas for electrostatic precipitation. The system of U.S. Pat. No. 4,779,207 includes a source of an SO.sub.3 conditioning agent, a means for controllably adding the conditioning agent to the flue gas, a means for detecting the input power level of the electrostatic precipitators and control means for monitoring the input power level and controlling the amount of conditioning agent added to the gas to substantially maintain input power to the electrostatic precipitator to predetermined levels.
Other conditioning systems are shown, for example, in U.S. Pat. Nos. 3,686,825; 4,042,348; 4,333,746; 4,466,815 and 4,533,364.